JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 21, NOVEMBER 1, 2014

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Distributed Brillouin Sensing With Centimeter-Order
Spatial Resolution in Polymer Optical Fibers
Neisei Hayashi, Yosuke Mizuno, Member, IEEE, and Kentaro Nakamura, Member, IEEE

Abstract—We present the first demonstration of distributed
strain/temperature sensing with a high spatial resolution in plastic
optical fibers (POFs) based on Brillouin optical correlation-domainreflectometry. A 50-cm-long strain-applied (or heated) POF section
is clearly detected with a theoretical spatial resolution of 34 cm, ahigh sampling rate of 3.3 Hz (per measured point), and a highsignal-to-noise ratio. A 10-cm-long heated POF section is also successfully detected with a theoretical resolution of 7.4 cm. The performance limitation of this system is then discussed.
Index Terms—Brillouin optical correlation-domain reflectometry (BOCDR), Brillouin scattering, polymer optical fibers, distributed measurement, optical fiber sensing.

I. INTRODUCTIONRILLOUIN scattering in optical fibers has been the subjectof extensive research for the past several decades because
it provides a means of measuring strain and temperature in a
distributed manner [1]–[5]. Up to now, only glass optical fibers
have been used for the sensor heads of Brillouin sensors, but they
are quite fragile and cannot withstand strains of several percent.
One attractive solution to this problem is to make use of plastic
optical fibers (POFs), which have such a high flexibility that they
can withstand large strains of several tens of percent [6]. Anotheradvantage of POF-based sensors is a unique function called a“memory effect” [7], with which the information on the appliedlarge strain can be stored owing to their plastic deformation.
Previous experimental studies on Brillouin scattering in POFs
[8]–[10] have revealed its potential applicability to large-strain
sensing [9] and to high-precision temperature sensing with lesssensitivity to strain [10].
Very recently, Minardo et al. [11] have demonstrated lowresolution distributed temperature sensing in a POF basedon Brillouin optical frequency-domain analysis (BOFDA) [3].
They detected a 4-m-long heated section—located at one end—of a 20-m-long POF, but the spatial resolution and signal-tonoise ratio (SNR) were not sufficiently high for practical use;
relatively high cost of the devices such as a vector network an-

B

Manuscript received April 15, 2014; revised July 4, 2014; accepted July 13,
2014. Date of publication July 31, 2014; date of current version September 17,
2014. This work was supported in part by a Grant-in-Aid for Young Scientists(A) (No. 25709032) from the Japan Society for the Promotion of Science (JSPS),
and by research grants from the General Sekiyu Foundation, the Iwatani Naoji
Foundation, and the SCAT Foundation. The work of N. Hayashi was supportedby the Grant-in-Aid for JSPS Fellows (No. 25007652).
The authors are with the Precision and Intelligence Laboratory, Tokyo
Institute of Technology, Kanagawa 226-8503, Japan (e-mail: hayashi@sonic.
pi.titech.ac.jp; ymizuno@sonic.pi.titech.ac.jp; knakamur@sonic.pi.titech.ac.
jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2014.2339361

alyzer and a microwave generator is also a problem. Althougha 3-cm spatial resolution has been obtained by BOFDA [12] ina silica single-mode fiber (SMF), such a high resolution has notbeen achieved in a POF not only because of the high propagation loss but also because of the weak Brillouin signal resulting
from its large core diameter and multimode nature.
In this work, we report on the first demonstration of distributed strain and temperature sensing with a centimeterorder spatial resolution in a POF based on Brillouin optical correlation-domain reflectometry (BOCDR) [5], which ishighly cost-effective. A 10-cm-long heated section—locatedaway from both ends—of a 1.3-m-long POF is successfully
detected with a theoretical spatial resolution of 7.4 cm and asampling rate of 3.3 Hz per measured point (corresponding to ameasurement time of ∼1 min, if the number of measured points
is 200). We also discuss how the characteristics of POFs (Brillouin frequency shift (BFS), Brillouin bandwidth, propagation
loss, etc.) affect the sensing performance of BOCDR.
II. PRINCIPLE
When light is propagating in an optical fiber, it is partiallyreturned via spontaneous Brillouin scattering. The backscattered
Stokes light spectrum is called Brillouin gain spectrum (BGS)[13], and the central frequency of the BGS is downshifted fromthe incident frequency by the amount called BFS. The BFS is
known to be ∼10.8 GHz for a silica SMF [13] and ∼2.8 GHz
for a perfluorinated graded-index (PFGI-) POF [8] at 1.55 μm.
If temperature change (or strain) is applied to the fiber, the
BFS shifts toward higher or lower frequency according to thefiber core material, in which lies the basic principle of Brillouintemperature (or strain) sensing. The temperature dependencecoefficient of the BFS is reported to be ∼1.0 MHz/K for a silica
SMF [14], and −3.2 MHz/K for a PFGI-POF [15] at 1.55 μm.
Considering that the strain dependence coefficient of the BFS ina PFGI-POF is −121.8 MHz/% [10], the absolute value of which
is about one fifth of that in a silica SMF, Brillouin scattering ina PFGI-POF has a big potential for high-precision temperaturesensing with reduced strain sensitivity [10].
Several distributed measurement techniques based on
Brillouin scattering in optical fibers have been proposed so far,which are classified into two categories: “reflectometry” and“analysis.” In reflectometry, based on spontaneous Brillouinscattering, a light beam is injected only into one end of thefiber; whereas in analysis, based on stimulated Brillouin scattering (SBS), two light beams are injected into both ends of thefiber. Analysis systems proposed so far include Brillouin optical time-, frequency-, and correlation-domain analysis (BOTDA[1], [16], [17], BOFDA [3], [11], [12], BOCDA [4], [18], [19]),0733-8724 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications standards/publications/rights/index.html for more information.

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in which a relatively large signal and thus a high SNR can beobtained. Two-end access is, however, less convenient, because
the system does not work completely when the fiber has even one
breakage point. Moreover, expensive devices are often required
to prepare so-called probe light to induce SBS. In contrast, eventhough the signal is weak, reflectometry such as Brillouin optical
time- and correlation-domain reflectometry (BOTDR [2], [20],[21], BOCDR [5], [22], [23]) can resolve these problems. As theinterface between a silica SMF and a POF is easily damaged byinjecting short optical pulses with high peak power [24], [25],here we focus on BOCDR.
First proposed in 2008 [5], BOCDR has been used as apromising distributed sensing technique with one-end accessibility, a high spatial resolution, a high sampling rate (i.e., fastmeasurement speed), and cost efficiency. Its operating principle is based on the correlation control of continuous lightwaves[26]; namely, the pump light and the reference light in a standard
self-heterodyne scheme for analyzing Brillouin signals [8] aresinusoidally frequency-modulated at fm , producing periodical
correlation peaks in the fiber to be measured. The measurement
range dm is determined by their interval, which is inversely
proportional to fm as
c
dm =
(1)
2nfm
where c is the velocity of light in vacuum and n is the refractive
index of the fiber core. By sweeping fm , the correlation peak,i.e., the sensing position, can be scanned along the fiber toacquire BGS or BFS distribution. According to theory [23],when fm is lower than the Brillouin bandwidth ΔνB , the spatial
resolution Δz is given by
Δz =

cΔνB
2πnfm Δf(2)

where Δf is the modulation amplitude of the optical frequency.
Considering that fm higher than ΔνB does not contribute to theenhancement of Δz [23], and that Δf is practically limited to ahalf of BFS (νB ) of the fiber because of the Rayleigh noise [5],[23], the limitation of the spatial resolution Δzm in is given bycΔzm in =.
(3)
πnνBThe number of effective sensing points NR , which can be regarded as an evaluation parameter of the system, is given by theratio of dm to Δz, as
NR =

πΔf
dm
=
.
Δz
ΔνB

(4)

To obtain higher NR , Δf needs to be raised but it should belower than νB /2; NR is thus limited to
πνB
.
(5)
NR m ax =2ΔνBIII. EXPERIMENTAL SETUP
POFs employed in the experiment were PFGI-POFs [27]with a numerical aperture of 0.185, a core diameter of 50 μm,
a cladding diameter of 100 μm, an overcladding diameter of
500 μm, a core refractive index of ∼1.35, and a propagation

Fig. 1. Schematic setup of BOCDR. EDFA, erbium-doped fiber amplifier;
ESA, electrical spectrum analyzer; FG, function generator; PC polarization
controller; PD, photo detector.

loss of ∼250 dB/km at 1.55 μm. The core/cladding layers andthe overcladding layer were composed of amorphous perfluorinated polymer and polycarbonate, respectively.
The schematic setup of BOCDR for distributed measurement
in a POF is shown in Fig. 1, which is basically the same asthat previously reported in [5]. All the optical paths except the
POF were silica SMFs. A distributed-feedback laser diode at
1.55 μm with 1-MHz linewidth was used as a light source,
and its output frequency was sinusoidally modulated by directmodulation of the driving current. Its output was divided into
two light beams with a coupler. One was directly used as thereference light of heterodyne detection, after passing througha 1-km delay fiber to adjust the correlation peak order, and an
erbium-doped fiber amplifier (EDFA) to enhance the beat signal.
The other beam was amplified with another EDFA, and injected
into the POF as the pump light (incident power: 27 dBm). The
optical beat signal between the Stokes light and the reference
light was then converted to an electrical signal with a photodetector (PD), which was finally monitored with an electrical
spectrum analyzer (ESA) with a 300-kHz frequency resolution.
Polarization state was optimized with a polarization controller(PC) at the beginning of each distributed measurement so that
the Rayleigh noise was minimal [28].
IV. EXPERIMENTAL RESULTS
A. Distributed Strain and Temperature Measurement
With High SNRFirst, we demonstrate distributed strain and temperature sensing with a moderate spatial resolution but with a high SNR.
The modulation frequency fm was swept from 11.654 to
11.698 MHz, corresponding to the measurement range dm of
9.5 m according to Eq. (1). The modulation amplitude Δf was
set to 0.9 GHz, resulting in the theoretical spatial resolution
Δz of 34 cm from Eq. (2) (the Brillouin bandwidth ΔνB is∼100 MHz [8] in a POF). Their ratio NR was 28. The 56th
correlation peak was used. The overall sampling rate of singlelocation measurement was 3.3 Hz.
Fig. 2 shows the structure of a 2-m-long POF to be measured, in which strains of <1.2% (within elastic region [9]) were

HAYASHI et al.: DISTRIBUTED BRILLOUIN SENSING WITH CENTIMETER-ORDER SPATIAL RESOLUTION IN POLYMER OPTICAL FIBERS

Fig. 2.

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Structure of POF under test (1).
Fig. 4.

Structure of POF under test (2).

The measured BFS distribution when temperature waschanged is also shown in Fig. 3(b), where the 50-cm-long heatedsection was clearly detected. The measurement time was also
about 1 min. The proportionality constant of temperature dependence was −3.27 MHz/°C, which is in good agreement withprevious result (−3.2 MHz/% [15]). The temperature measurement error was evaluated to be 3.1 °C.
B. Distributed Temperature Measurement With HighSpatial Resolution

Fig. 3. Measured BFS distributions with a 50-cm-long section (a) strainapplied, and (b) heated.

Next, we demonstrate distributed temperature sensing with acentimeter-order spatial resolution. The modulation configurations of the light source were: fm = 53.321–53.451 MHz andΔf = 0.9 GHz, corresponding to dm of 2.1 m and Δz of 7.4 cm(NR = 28). Fig. 4 shows the structure of a 1.3-m-long POF
employed, where a 10-cm-long section was heated to 40 °C.
Fig. 5(a) shows the measured distribution of normalized BGS
along the POF, and Fig. 5(b) shows the BGS examples at nonheated and heated positions (relative positions of 67 and 104 cm,
respectively). Fig. 5(c) shows the BFS distribution corresponding to Fig. 5(a). The measurement time was approximately 40 s
(130 points). The BFS clearly downshifted at the 10-cm-longheated section. The amount of the BFS shift was approximately
26 MHz, which agrees well with the actual temperature (40 °C).
The gradual BFS changes at the relative positions of ∼90 and∼115 cm were probably caused by the overlap of two broad
BGSs (ΔνB ∼ 100 MHz) from the sections with and without
temperature changed.
C. Discussion on Sensor Performance

applied to a 50-cm-long section fixed on a translation stage, orthe same section was heated up to 65 °C (sufficiently lower thanglass-transition temperature [29]). One end of the POF was buttcoupled to a silica SMF (second port of the circulator) via an
SC connector, and the other end was cut at 8° to suppress theFresnel reflection. The room temperature was 18 °C.
The measured BFS distribution when strain was appliedis shown in Fig. 3(a). The measurement time was approximately 1 min (200 points), which can be set shorter byreducing the measured points. The 50-cm-long strain-applied
section was successfully detected. The BFS shifted to lower
frequency with increasing strain with a proportionality constantof −115.3 MHz/%, which was moderately consistent with that
previously reported (−121.8 MHz/% [10]. The BFS changed
even along the strain-free sections by about ±10 MHz, whichindicates that the strain measurement error is ±0.09%.

Finally, we compare the performances of POF-based BOCDR
with those of silica SMF-based BOCDR. First, according to Eq.
(3), the highest spatial resolution Δzm in theoretically achievablein POF-based BOCDR (νB ∼ 2.8 GHz; n ∼ 1.35) is calculated
to be 23 mm, which is approximately 1/4 of that in SMF-based
BOCDR (νB ∼ 10.8 GHz; n ∼ 1.46). However, a weak Brillouin signal in a POF [8]–[10], leading to a low SNR, practicallylimits the spatial resolution, as shown in the aforementioned experiment. Next, according to Eq. (5), the maximal number ofeffective sensing points NR m ax of POF-based BOCDR (ΔνB
∼100 MHz) is calculated to be 44, which is ∼1/13 of thatof SMF-based BOCDR (ΔνB ∼ 30 MHz). This problem canbe mitigated by employing so-called temporal-gating [30] anddouble-modulation schemes [31]. Note that the measurement
range dm itself is limited not only by its trade-off relation to Δz
but also by the high propagation loss (250 dB/km at 1.55 μm) of4002

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V. CONCLUSION
Distributed strain and temperature sensing with a centimeterorder spatial resolution in a POF was demonstrated for the firsttime using a cost-effective BOCDR technique. A 10-cm-longheated section of a 1.3-m-long POF was successfully detectedwith a theoretical spatial resolution of 7.4 cm and a sampling
rate of 3.3 Hz (per measured point). The limitation of the sensing performances was discussed. We believe that our results
have overcome a stereotype of perceiving POF-based Brillouin
distributed sensing with a high resolution and/or a high SNR as
almost infeasible.
ACKNOWLEDGMENT
The authors are grateful to Profs. K. Hotate and M. Kishi, theUniversity of Tokyo, Japan, for their experimental assistance.
REFERENCES

Fig. 5. (a) Normalized BGS distribution. (b) Examples of BGS (Z 1 : at 67 cm(room temperature); Z 2 : at 104 cm (heated)). (c) BFS distribution measured
with cm-order resolution.

the POF. Currently, the practical limitation of dm is several tensof meters (depending on Δz, incident power, and many other parameters); we believe it can be elongated to several hundreds ofmeters by using shorter pump wavelengths at which the propagation loss is much lower (for instance, ∼10-dB/km loss isreported at 0.98 μm [27]). As for the sampling rate of singlelocation measurement, 3.3 Hz demonstrated in the experimentis restricted by the speed of signal acquisition from the ESA
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enhanced by use of faster data acquisition methods that havebeen implemented in SMF-based BOCDR [22] and BOCDA[32]. Highly accurate discriminative sensing of strain and temperature [33] using POFs is another important problem to be
tackled.

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Neisei Hayashi was born in Gunma, Japan, on April 13, 1988. He received the
B.E. degree in advanced production from the Gunma National College Technology (GNCT), Gunma, Japan, in 2011, and the M.E. degree in electronicengineering from Tokyo Institute of Technology, Tokyo, Japan, in 2013.
From 2008 to 2010, he studied a ring-wandering phenomenon for the Bachelor’s degree at the GNCT. From 2011 to 2013, he worked on nonlinear optics
in polymers for the Master’s degree at Tokyo Institute of Technology. In 2013,
he became a Research Fellow (DC1) of the Japan Society for the Promotion of
Science, and, since then, he has been continuing to study polymer optics for the
Dr. Eng. degree at Tokyo Institute of Technology. His research interests include
fiber-optic sensors, polymer optics, and ultrasonics.
Mr. Hayashi is the Winner of the TELECOM System Technology Award
for Student 2012. He is a Member of the Japanese Society of Applied Physics,
and the Institute of Electronics, Information, and Communication Engineers ofJapan.

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Yosuke Mizuno was born in Hyogo, Japan, on October 13, 1982. He received
the B.E., M.E., and Dr. Eng. degrees in electronic engineering from the University of Tokyo, Tokyo, Japan, in 2005, 2007, and 2010, respectively. From 2007
to 2010, he was engaged in Brillouin optical correlation-domain reflectometry
for the Dr. Eng. degree at the University of Tokyo.
From 2007 to 2010, he was a Research Fellow of the Japan Society for the
Promotion of Science. From 2010 to 2012, as a Research Fellow of JSPS, he
worked on polymer optics at Tokyo Institute of Technology, Japan. In 2011, hestayed at BAM Federal Institute for Materials Research and Testing, Germany,
as a Visiting Research Associate. Since 2012, he has been an Assistant Professor at the Precision and Intelligence Laboratory, Tokyo Institute of Technology,
where he is active in fiber-optic sensing, polymer optics, and ultrasonics.
Dr. Mizuno received the Funai Research Award 2010, the Ando Incentive
Prize for the Study of Electronics 2011, the NF Foundation R&D Encouragement Award 2012, and the Kenjiro Nakamura Prize 2012. He is a Member of
the Japanese Society of Applied Physics, and the Institute of Electronics, Information, and Communication Engineers of Japan.

Kentaro Nakamura (M’00) was born in Tokyo, Japan, on July 3, 1963. Hereceived the B.E., M.E., and Dr. Eng. degrees from Tokyo Institute of Technology, Tokyo, Japan, in 1987, 1989, and 1992, respectively.
Since 2010, he has been a Professor at the Precision and Intelligence Laboratory, Tokyo Institute of Technology. His research field is the applications ofultrasonic waves, measurement of vibration and sound using optical methods,
and fiber-optic sensing.
Prof. Nakamura is the Winner of the Awaya Kiyoshi Award for encouragement of research from the Acoustical Society of Japan in 1996, and the Best
Paper Award from the Institute of Electronics, Information and Communication Engineers in 1998, and from the Symposium on Ultrasonic Electronics in2007 and 2011, respectively. He also received the Japanese Journal of Applied
Physics Editorial Contribution Award from the Japan Society of Applied Physics
in 2007. He is a Member of the ASJ, the JSAP, the IEICE, and the Institute ofElectrical Engineers of Japan.